Protection device for preventing scouring of submarine pipeline and size calculation method of protection device

Abstract

The invention discloses a submarine pipeline scour prevention protection device and a size calculation method thereof.The submarine pipeline scour prevention protection device comprises a plurality of protection structures arranged at equal intervals in the axis direction of a pipeline, each protection structure comprises at least two protection layers fixed together in an up-down stacking mode, and the protection layer on the bottommost layer comprises first geotechnical cloth and a plurality of first protection blocks; the weight of the bottommost protective layer is larger than that of the other protective layers, each of the other protective layers comprises second geotechnical cloth, a plurality of second protective blocks and a plurality of third protective blocks, the second protective blocks and the third protective blocks are at least different in height size, and the second protective blocks and the third protective blocks are fixed to the second geotechnical cloth in an alternate arrangement mode. According to the protection structure designed by the invention, the resistance of ocean current flowing through the pipeline can be increased in a deep sea area, when the water current flows through the protection structure, the flow speed of the water current can be slowed down, local backflow vortexes are formed, the scouring effect of the water current is reduced, and better siltation promotion capability is achieved.

Description

Technical Field

[0001] The invention relates to the technical field of submarine pipeline scour protection, and in particular to a protective device for submarine pipeline scour protection suitable for deep water and a size calculation method thereof. Background Art

[0002] Submarine pipelines are an important component of marine oil and gas transportation equipment. Maintaining the safety of submarine pipelines is an important guarantee for ensuring the development of marine gas fields and protecting the marine environment. Although the speed of the ocean current in deep water areas is relatively low, the soil in the shallow layers of deep water is mostly soft, and there are many uneven areas on the seabed. Deepwater submarine pipelines are prone to pipeline suspension under the scouring of ocean currents. When the ocean current flows through the suspended pipeline, vortex shedding will occur in the wake area. When the frequency of vortex shedding is close to the natural frequency of the pipeline, vortex-induced vibration will occur in the pipeline, which will cause fatigue damage to the pipeline and trigger marine oil and gas leakage accidents.

[0003] The existing pipeline scour protection methods mainly include the spoiler method, the bionic seaweed method, the riprap protection method, and the concrete soft drainage method. The spoiler method realizes the scour protection of the pipeline by making the pipeline self-buried. Since deep-water areas are mostly soft clay, this method is not suitable for deep-water areas; the bionic seaweed method uses bionic seaweed to slow down the scouring of the seabed by water flow. This method is time-consuming and high-investment, and is not suitable for the scouring protection of deep-water long-distance submarine pipelines; the riprap protection method reduces the scouring of the seabed soil by water flow by covering the gravel layer. This method has the problem of stone stability, and with the increase of water depth, the cost of deep-water riprap increases sharply; the concrete soft drainage method protects the pipeline through concrete interlocking pieces and geotextiles, but the gaps between concrete blocks are large, and the lower part of the soft drainage will experience secondary scouring after silting.

[0004] Therefore, there is an urgent need for a protective device suitable for anti-scouring of submarine pipelines in deep water and a size calculation method thereof to solve the above technical problems. Summary of the invention

[0005] The present invention aims to solve the above technical problems, namely, to solve the problem that existing submarine pipelines in deep water areas will span under the scouring action of ocean currents, and the spanning pipelines are prone to fatigue damage, while the current pipeline scouring protection methods are not suitable for pipeline scouring protection in deep water areas.

[0006] To this end, in a first aspect, the present invention provides a protective device for preventing scour of a submarine pipeline, comprising a plurality of protective structures arranged at equal intervals along the axial direction of the submarine pipeline, the protective structure comprising at least two protective layers fixed together in an upper and lower stacking manner, the bottom protective layer comprising a first geotextile and a plurality of first protective blocks fixed on the first geotextile, the weight of the bottom protective layer being greater than the weight of the protective layers of other layers, the protective layers of other layers comprising a second geotextile, a plurality of second protective blocks and a plurality of third protective blocks, the second protective blocks and the third protective blocks having at least a height dimension different in size, and the second protective blocks and the third protective blocks being fixed on the second geotextile in an alternating arrangement.

[0007] In a specific embodiment of the above-mentioned protective device for submarine pipeline anti-scouring, multiple first protective blocks are fixed on the first geotextile in a closely arranged manner, and the second protective blocks and the third protective blocks are fixed on the second geotextile in an alternating closely arranged manner.

[0008] In a specific embodiment of the above-mentioned protective device for preventing scour of submarine pipelines, the size of the first protective block is larger than the size of the second protective block and the third protective block.

[0009] In a specific embodiment of the above-mentioned protective device for preventing scour of submarine pipelines, the shape of the first protective block is a quadrangular pyramid shape or a pentagonal pyramid shape.

[0010] In a specific embodiment of the above-mentioned protective device for submarine pipeline anti-scouring, the second protective block and the third protective block are both in the shape of a quadrangular pyramid or a pentagonal pyramid.

[0011] In a specific embodiment of the above-mentioned protective device for preventing submarine pipeline scour, the first protective block is made of concrete, and the second protective block and the third protective block are both made of asphalt.

[0012] In a specific embodiment of the above-mentioned protective device for preventing scour of submarine pipelines, the protective structure includes three protective layers fixed together in an upper and lower stacking manner, the size of the second protective block in the upper protective layer is less than or equal to the size of the second protective block in the lower protective layer, and the size of the third protective block in the upper protective layer is less than or equal to the size of the third protective block in the lower protective layer.

[0013] In a second aspect, the present invention further provides a method for calculating the size of a protective device for preventing scour of a submarine pipeline according to any one of the first aspects, wherein the protective device for preventing scour of a submarine pipeline is, and the method for calculating the size comprises the following steps: S1, using the scour calculation numerical model to calculate the scour parameters corresponding to multiple groups of data parameters, and grouping some of the data parameters and the corresponding scour parameters as training set parameters, and grouping the remaining parameters as test set parameters; wherein each group of data parameters includes the size parameters of the first protection block, the second protection block and the third protection block and the spacing parameters between two adjacent protection structures, and the scour parameters include the scour velocity, the scour area and the scour area; S2, training the neural network based on the training set parameters to obtain neural network parameters; S3, building a neural network model according to the neural network parameters; S4, inputting the data parameters in the test set parameters obtained by the scour calculation numerical model into the neural network model to obtain the corresponding predicted scour velocity, predicted scour depth and predicted scour area; S5, obtaining the error value between the scouring velocity, scouring area and scouring area in the test set parameters and the corresponding predicted scouring velocity, predicted scouring depth and predicted scouring area; S6, determining the size of the error value and the preset difference value; S7, determining an approximate proxy model based on a judgment result between the error value and a preset difference value; S8, taking the minimum values ​​of scour area, scour depth and total cost as the optimization objective; S9, establishing a multi-objective optimization mathematical model based on the set constraints and the optimization objectives; S10, using an approximate proxy model and NSGA-Ⅱ algorithm to solve the Pareto frontier of the multi-objective optimization mathematical model under the set ocean current velocity probability distribution, and comparing different solutions in the Pareto frontier, finally obtaining the size parameters of the first protection block, the second protection block and the third protection block that meet the requirements and the spacing parameters between two adjacent protection structures.

[0014] In the specific implementation of the size calculation method of the protective device for submarine pipeline anti-scour, the step of "using the approximate proxy model and NSGA-Ⅱ algorithm to solve the Pareto front of the multi-objective optimization mathematical model under the set ocean current velocity probability distribution" specifically includes: The approximate proxy model is used to obtain mapping relationships between the scouring area and the scouring depth and the size parameters of the first protection block, the second protection block and the third protection block, and the spacing parameters between two adjacent protection structures; A mapping relationship between the total cost and the size parameters of the first protection block, the second protection block and the third protection block and the spacing parameters between two adjacent protection structures is established by using a total cost calculation formula; These mapping relationships are substituted into the NSGA-Ⅱ algorithm to solve the Pareto frontier of the multi-objective optimization mathematical model under the set ocean current velocity probability distribution.

[0015] In a specific implementation of the above-mentioned method for calculating the size of the protective device for submarine pipeline anti-scour, the step of "determining the approximate proxy model based on the judgment result of the error value and the preset difference value" specifically includes: If the error value is less than or equal to a preset difference value, the neural network model is selected as an approximate proxy model; If the error value is greater than the preset difference value, steps S2-S6 are repeated until the error value is less than or equal to the preset difference value.

[0016] Compared with the prior art, the present invention has the following beneficial effects: 1. The weight of the bottom protective layer is designed to be greater than that of the upper protective layer, which can ensure the overall stability of the protective structure, and the upper protective layer is designed to be lighter in weight to reduce the difficulty of transportation and installation; second protective blocks and third protective blocks of different sizes are arranged in the other protective layers except the bottom layer, and are arranged in an alternating manner, which increases the resistance of the ocean current flowing through the submarine pipeline. When the water flows through the protective structure, the water flow rate will be slowed down and a local backflow vortex will be formed, which can effectively reduce the scouring effect of the water flow and has a good siltation promoting ability. Compared with the existing protection method, the present invention has low construction difficulty and low cost, is suitable for scouring protection of deep-water long-distance submarine pipelines, and can avoid secondary scouring after siltation promotion.

[0017] 2. This scour protection device has many size parameters, and it is difficult to establish the mapping relationship between the scour area, scour depth and the size parameters of the first protection block, the second protection block and the third protection block, as well as the spacing parameters between two adjacent protection structures. This size calculation method uses a machine learning method to establish an approximate proxy model to describe the mapping relationship between the scour area, scour depth and the size parameters of the first protection block, the second protection block and the third protection block, as well as the spacing parameters between two adjacent protection structures. A multi-objective optimization mathematical model for size parameters is established with the scour area, scour depth and minimum total cost as the optimization objectives, and the NSGA-Ⅱ algorithm is used to solve it. The optimal size parameters of the protective device finally obtained take into account the minimum scour area, the minimum scour depth and the minimum total cost at the same time. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The preferred embodiments of the present invention are described below in conjunction with the accompanying drawings, in which: Figure 1 It is a schematic structural diagram of a protective device for preventing scour of a submarine pipeline provided by the present invention; Figure 2 yes Figure 1 A magnified view of the middle protection structure; Figure 3 is a dimension parameter marking diagram of the first guard block; Figure 4 is a dimension parameter marking diagram of the second guard block; Figure 5 It is the dimension parameter marking diagram of the third protection block.

[0019] List of reference numerals: 1. Submarine pipeline; 2. First protection block; 3. Second protection block; 4. Third protection block. DETAILED DESCRIPTION

[0020] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0021] In the description of the present invention, it should be noted that the directions or positional relationships indicated by the terms "upper", "lower", "inner", "outer", etc. are based on the directions or positional relationships shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the system or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", etc. are used to define components, only for the convenience of distinguishing the above components, and unless otherwise stated, the above terms have no special meanings and cannot be understood as indicating or implying relative importance.

[0022] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installation", "setting", and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be an indirect connection through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0023] The present invention relates to the technical field of submarine pipeline scouring protection, and in particular to a protective device for submarine pipeline scouring protection and a method for calculating its dimensions. The purpose is to solve the problem that submarine pipelines in existing deep-water areas will span under the scouring action of ocean currents, and that spanning pipelines are prone to fatigue damage, while the current submarine pipeline scouring protection methods are not suitable for pipeline scouring protection in deep-water areas. To this end, the present invention provides a protective device for submarine pipeline scouring protection, comprising a plurality of protective structures arranged at equal intervals along the axis of the submarine pipeline, the protective structure comprising at least two layers of protective layers fixed together in an up-and-down stacking manner, the bottom protective layer comprising a first geotextile and a plurality of first protective blocks fixed on the first geotextile, the bottom protective layer weighing more than the other layers, the other layers of protective layers comprising a second geotextile, a plurality of second protective blocks and a plurality of third protective blocks, the second protective blocks and the third protective blocks having at least a different height dimension, and the second protective blocks and the third protective blocks having different height dimensions. The three protection blocks are fixed on the second geotextile in an alternating manner. The weight of the bottom protection layer is designed to be greater than that of the upper protection layer, which can ensure the overall stability of the protection structure. The upper protection layer is designed to be lighter in weight to reduce the difficulty of transportation and installation. Second protection blocks and third protection blocks of different sizes are set in the other protection layers except the bottom layer, and are arranged in an alternating manner, which increases the resistance of the ocean current flowing through the submarine pipeline. When the water flows through the protection structure, the water flow velocity will be slowed down and a local backflow vortex will be formed, which can effectively reduce the scouring effect of the water flow and has a good siltation-promoting ability.

[0024] Next, the protective device for preventing scour of submarine pipelines and the method for calculating the size thereof provided by the embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0025] See also Figure 1-2 The present invention provides an anti-scour protection device for a submarine pipeline 1, comprising a plurality of protection structures arranged at equal intervals along the axial direction of the submarine pipeline 1, the protection structure comprising at least two protective layers fixed together in an up-and-down stacking manner, the bottom protective layer comprising a first geotextile and a plurality of first protection blocks 2 fixed on the first geotextile, the weight of the bottom protective layer being greater than the weight of the protection layers of other layers, the protection layers of other layers comprising a second geotextile, a plurality of second protection blocks 3 and a plurality of third protection blocks 4, at least the height dimension of the second protection blocks 3 and the third protection blocks 4 being different, the second protection blocks 3 and the third protection blocks 4 being fixed on the second geotextile in an alternating arrangement.

[0026] Specifically, a plurality of first protective blocks 2 are fixed on the first geotextile in a closely arranged manner, and the second protective blocks 3 and the third protective blocks 4 are fixed on the second geotextile in an alternating closely arranged manner. When installing on the seabed, the entire protective structure can be laid on the submarine pipeline 1. Since the second protective blocks 3 and the third protective blocks 4 are arranged alternately, when the ocean current flows through the submarine pipeline 1, the resistance of the ocean current flowing through the submarine pipeline 1 is increased, the flow rate of the water flow can be slowed down and a local backflow vortex can be formed, which reduces the scouring effect of the water flow, and can cause the silt in the ocean current to settle on the protective structure, and has a good silt-promoting ability. The upper protective layer and the lower protective layer are fixed by bonding. For example, Figure 1 As shown, the protective layer consists of two layers, and the second geotextile on the upper layer is fixed to the top of the first protective block 2 on the lower layer by bonding.

[0027] In the above embodiment, the second protection block 3 and the third protection block 4 may differ in size only in height, such as Figure 1 As shown, the height and another or two size parameters may be different, which is not specifically limited in this application and can be flexibly set according to actual conditions without deviating from the basic principles of the present invention. The second and third protective blocks are of different sizes, which can increase the friction coefficient of the surface, thereby increasing the friction force, and then increase the resistance of the sea current flowing through the submarine pipeline, slowing down the flow rate of the water flow, and at the same time having good silt-promoting ability. In one embodiment, see Figure 2 The size of the first protective block 2 is larger than that of the second protective block 3 and the third protective block 4. This helps to increase the weight of the bottom protective layer and ensures the stability of the overall protective structure laid on the submarine pipeline 1.

[0028] In this application, see Figure 2 , the gaps between the protection blocks in the upper and lower protection layers are in a staggered state.

[0029] In one embodiment, the first protection block 2 is in the shape of a quadrangular pyramid or a pentagonal pyramid, or may be in the shape of a hexagonal pyramid or a triangular pyramid.

[0030] In one embodiment, the second protection block 3 and the third protection block 4 are both in the shape of a quadrangular pyramid or a pentagonal pyramid, or in the shape of a hexagonal pyramid or a triangular pyramid.

[0031] In one embodiment, the first protective block 2 is made of concrete, and the second protective block 3 and the third protective block 4 are both made of asphalt. The first protective block 2 is designed to be made of concrete, which has a high density and can ensure the stability of the protective structure, while the second protective block 3 and the third protective block 4 on the upper layer are made of asphalt, which has a low density and can reduce the difficulty of transportation and installation.

[0032] In one embodiment, the protective structure includes three protective layers fixed together in an upper and lower stacking manner, the size of the second protective block 3 in the upper protective layer is less than or equal to the size of the second protective block 3 in the lower protective layer, and the size of the third protective block 4 in the upper protective layer is less than or equal to the size of the third protective block 4 in the lower protective layer.

[0033] In another embodiment, the present invention further provides a method for calculating the size of a protective device for submarine pipeline anti-scour, the protective device for submarine pipeline anti-scour is the protective device for submarine pipeline anti-scour described in the above various embodiments, and the size calculation method comprises the following steps: S1, using the scour calculation numerical model to calculate the scour parameters corresponding to multiple sets of data parameters, and grouping some of the data parameters and the corresponding scour parameters as training set parameters, and grouping the remaining parameters as test set parameters; wherein each set of data parameters includes the size parameters of the first protection block, the second protection block and the third protection block and the spacing parameters between two adjacent protection structures, and the scour parameters include the scour velocity, the scour area and the scour area; wherein, data with different values ​​are taken to form multiple sets of data parameters; S2, training the neural network based on the training set parameters to obtain neural network parameters, where the neural network parameters at least include weights and thresholds; S3, building a neural network model according to the neural network parameters; S4, inputting the data parameters in the test set parameters into the neural network model to obtain the corresponding predicted scour velocity, predicted scour depth and predicted scour area; S5, obtaining the error values ​​between the scouring velocity, scouring area and scouring area in the test set parameters and the corresponding predicted scouring velocity, predicted scouring depth and predicted scouring area; S6, determining the size of the error value and the preset difference value; S7, determining an approximate proxy model based on a judgment result between the error value and a preset difference value; S8, taking the minimum values ​​of scour area, scour depth and total cost as the optimization objective; S9, establishing a multi-objective optimization mathematical model based on the set constraints and optimization objectives; S10, using the approximate proxy model and NSGA-Ⅱ algorithm to solve the Pareto frontier of the multi-objective optimization mathematical model under the set ocean current velocity probability distribution, and compare different solutions in the Pareto frontier, and finally obtain the size parameters of the first protection block, the second protection block and the third protection block that meet the requirements and the spacing parameters between two adjacent protection structures. Since the obtained Pareto frontier contains a series of solutions for size parameters and spacing parameters, it is a set of solutions. In this set, some solutions are very extreme, so they need to be screened, and finally the size parameters and spacing parameters that meet the expected requirements for scour area, scour depth and total cost are obtained.

[0034] In step S1, the specific method of obtaining the scour calculation numerical model is to establish the scour calculation numerical model based on the fluid dynamics analysis software FLUENT or FLOW-3D, and then build a pipeline scour test platform, carry out pipeline scour tests and verify the accuracy of the numerical model from at least the perspectives of scour velocity, scour depth, scour area, and scour pit shape. Through the scour test platform, multiple groups of experimental data on the perspectives of scour velocity, scour depth, scour area, and scour pit shape are obtained, and these data are compared with the data obtained by numerical simulation. The final required scour calculation numerical model can be obtained within the required error range.

[0035] Specifically, the step S7 of “determining the approximate proxy model based on the judgment result between the error value and the preset difference value” specifically includes: If the error value is less than or equal to the preset difference value, the neural network model is selected as the approximate proxy model; If the error value is greater than the preset difference value, steps S2-S6 are repeated until the error value is less than or equal to the preset difference value. The preset difference value may be set to 1%.

[0036] In step S10, the step of "using the approximate proxy model and the NSGA-Ⅱ algorithm to solve the Pareto frontier of the multi-objective optimization mathematical model under the set ocean current velocity probability distribution" specifically includes: The mapping relationship between the scouring area and the scouring depth and the size parameters of the first protection block, the second protection block and the third protection block and the spacing parameters between two adjacent protection structures is obtained by using an approximate proxy model; A mapping relationship between the total cost and the size parameters of the first protection block, the second protection block and the third protection block and the spacing parameters between two adjacent protection structures is established by using a total cost calculation formula; These mapping relationships are substituted into the NSGA-Ⅱ algorithm to solve the Pareto frontier of the multi-objective optimization mathematical model under the set ocean current velocity probability distribution.

[0037] Specifically, the total cost calculation formula is: the total cost is the sum of the manufacturing cost and the installation cost, where the installation cost is the manufacturing cost multiplied by a coefficient, which is less than one, and the manufacturing cost is the product of the total volume, the unit volume cost and the total number of protective structures, the total volume is obtained based on the size parameters, and the total number of protective structures is obtained based on the spacing parameters between two adjacent protective structures and the pipeline length.

[0038] Below Figure 1 The protective layer shown in the figure is two layers, the first protective block in the lower layer is made of concrete, and the second and third protective blocks in the upper layer are made of asphalt. The first, second and third protective blocks are all in the shape of a quadrangular pyramid, and the second and third protective blocks have different heights, but the bottom length and bottom width are the same. Figure 1 and Figure 3-5 As shown, the size parameters of the first protective block include the bottom length , bottom width , top side length , top width ,high The size parameters of the second protection block include the bottom length , bottom width , top side length , top width ,high , the size parameters of the third protection block include the bottom length , bottom width ,high , the spacing between adjacent protective layers is . Execute step S1, take multiple groups of different data to form multiple groups of data parameters, calculate the corresponding scour parameters based on these data parameters, and then execute steps S2-S7. Let the scour area be S, the scour depth be h, and the total cost of the protective structure be C. At this time, the lowest value of S, h, and C is taken as the optimization goal. Since the manufacturing process is related to the mass, and the mass is the product of volume and density, the manufacturing process is also related to the size parameters and spacing. Therefore, the optimization goals are as follows: .

[0039] The constraints set are as follows: .

[0040] A multi-objective optimization mathematical model is established according to the set constraints and optimization objectives, and after executing steps S8-S9, step S10 is executed. The NSGA-Ⅱ algorithm in step S10 for solving the multi-objective optimization mathematical model is a prior art known to those skilled in the art, and therefore, it will not be described in detail here, which does not affect the sufficiency of the disclosure of this application.

[0041] In the present application, there are many size parameters for the protective structure, and it is difficult to establish a mapping relationship between the scour area, scour depth and the size parameters of the first protective block, the second protective block and the third protective block, as well as the spacing parameters between two adjacent protective structures. This size calculation method uses a machine learning method to establish an approximate proxy model to describe the mapping relationship between the scour area, scour depth and the size parameters of the first protective block, the second protective block and the third protective block, as well as the spacing parameters between two adjacent protective structures. A multi-objective optimization mathematical model for size parameters is established with the scour area, scour depth and minimum total cost as the optimization objectives, and the NSGA-Ⅱ algorithm is used to solve it. The optimal size parameters of the protective device finally obtained take into account the minimum scour area, the minimum scour depth and the minimum total cost at the same time.

[0042] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the protection scope of the technical solutions of the embodiments of the present invention.

Claims

1. A protective device for submarine pipeline anti-scouring, characterized in that: The invention comprises a plurality of protective structures which are arranged at equal intervals along the axis direction of the submarine pipeline, wherein the protective structure comprises at least two protective layers which are fixed together in an upper and lower stacking manner, the bottom protective layer comprises a first geotextile and a plurality of first protective blocks fixed on the first geotextile, the weight of the bottom protective layer is greater than the weight of the protective layers of other layers, and the protective layers of other layers comprise a second geotextile, a plurality of second protective blocks and a plurality of third protective blocks, at least the height dimension of the second protective blocks and the third protective blocks is different, and the second protective blocks and the third protective blocks are fixed on the second geotextile in an alternating arrangement manner.

2. The protective device for preventing scouring of submarine pipelines according to claim 1 is characterized in that: A plurality of the first protection blocks are fixed on the first geotextile in a closely arranged manner, and the second protection blocks and the third protection blocks are fixed on the second geotextile in an alternately closely arranged manner.

3. The protective device for preventing scouring of submarine pipelines according to claim 1, characterized in that: The first protection block has a size greater than that of the second protection block and the third protection block.

4. The protective device for preventing scouring of submarine pipelines according to claim 1, characterized in that: The first protection block is in the shape of a quadrangular pyramid or a pentagonal pyramid.

5. The protective device for preventing scouring of submarine pipelines according to claim 1, characterized in that: The second protection block and the third protection block are both in the shape of a quadrangular pyramid or a pentagonal pyramid.

6. The protective device for preventing scour of submarine pipeline according to claim 1, characterized in that: The first protection block is made of concrete, and the second protection block and the third protection block are both made of asphalt.

7. The protective device for preventing scour of submarine pipeline according to claim 1, characterized in that: The protective structure includes three protective layers fixed together in an upper and lower stacking manner, the size of the second protective block in the upper protective layer is less than or equal to the size of the second protective block in the lower protective layer, and the size of the third protective block in the upper protective layer is less than or equal to the size of the third protective block in the lower protective layer.

8. A method for calculating the size of a protective device for submarine pipeline anti-scour according to any one of claims 1 to 7, characterized in that: The following steps are involved: S1, using the scour calculation numerical model to calculate the scour parameters corresponding to multiple groups of data parameters, and grouping some of the data parameters and the corresponding scour parameters as training set parameters, and grouping the remaining parameters as test set parameters; wherein each group of data parameters includes the size parameters of the first protection block, the second protection block and the third protection block and the spacing parameters between two adjacent protection structures, and the scour parameters include the scour velocity, the scour area and the scour area; S2, training the neural network based on the training set parameters to obtain neural network parameters; S3, building a neural network model according to the neural network parameters; S4, inputting the data parameters in the test set parameters into the neural network model to obtain the corresponding predicted scour velocity, predicted scour depth and predicted scour area; S5, obtaining the error value between the scour velocity, scour area and scour area calculated by the scour calculation numerical model in the test set parameters and the corresponding predicted scour velocity, predicted scour depth and predicted scour area; S6, determining the size of the error value and the preset difference value; S7, determining an approximate proxy model based on a judgment result between the error value and a preset difference value; S8, taking the minimum values ​​of scour area, scour depth and total cost as the optimization objective; S9, establishing a multi-objective optimization mathematical model based on the set constraints and the optimization objectives; S10, using an approximate proxy model and NSGA-Ⅱ algorithm to solve the Pareto frontier of the multi-objective optimization mathematical model under the set ocean current velocity probability distribution, and comparing different solutions in the Pareto frontier, finally obtaining the size parameters of the first protection block, the second protection block and the third protection block that meet the requirements and the spacing parameters between two adjacent protection structures.

9. The method for calculating the size of a protective device for preventing scour of a submarine pipeline according to claim 8, characterized in that: The steps of "using the approximate proxy model and NSGA-Ⅱ algorithm to solve the Pareto frontier of the multi-objective optimization mathematical model under the set ocean current velocity probability distribution" specifically include: The approximate proxy model is used to obtain mapping relationships between the scouring area and the scouring depth and the size parameters of the first protection block, the second protection block and the third protection block, and the spacing parameters between two adjacent protection structures; A mapping relationship between the total cost and the size parameters of the first protection block, the second protection block and the third protection block and the spacing parameters between two adjacent protection structures is established by using a total cost calculation formula; These mapping relationships are substituted into the NSGA-Ⅱ algorithm to solve the Pareto frontier of the multi-objective optimization mathematical model under the set ocean current velocity probability distribution.

10. The method for calculating the size of a protective device for preventing scour of a submarine pipeline according to claim 8, characterized in that: The step of "determining an approximate proxy model based on the judgment result between the error value and the preset difference value" specifically includes: If the error value is less than or equal to a preset difference value, the neural network model is selected as an approximate proxy model; If the error value is greater than the preset difference value, steps S2-S6 are repeated until the error value is less than or equal to the preset difference value.

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